The invention proceeds from oxygen-consuming electrodes known per se which are configured as sheet-like gas diffusion electrodes and usually comprise an electrically conductive support and a gas diffusion layer having a catalytically active component.
Various proposals for the operation of oxygen-consuming electrodes in electrolysis cells on an industrial scale are fundamentally known from the prior art. The basic idea is to replace the hydrogen-evolving cathode of the electrolysis (for example in chloralkali electrolysis) by the oxygen-consuming electrode (cathode). An overview of possible cell designs and solutions may be found in the publication by Moussallem et al., “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-consuming electrode, hereinafter also referred to as OCE for short, has to meet a series of requirements in order to be able to be used in industrial electrolysers. Thus, the catalyst and all other materials used have to be chemically stable to sodium hydroxide solution having a concentration of about 32% by weight and to pure oxygen at a temperature of typically 80-90° C. A high degree of mechanical stability is likewise required, since the electrodes are installed and operated in electrolysers having an electrode area of usually more than 2 m2 (industrial size). Further properties are: a high electrical conductivity, a low layer thickness, a high internal surface area and a high electrochemical activity of the electrocatalyst, and also impermeability so that gas and liquid spaces remain separated from one another. The long-term stability and low production costs are further particular requirements which an industrially usable oxygen-consuming electrode has to meet.
The invention relates in particular to an oxygen-consuming electrode which is built up in a plurality of layers and which has a difference in respect of the catalyst or PTFE concentration between the gas side and the electrolyte side and can be produced by the wet process.
Oxygen-consuming cathodes according to the prior art are used in various arrangements in electrochemical processes, for example in the generation of power in fuel cells or in the electrolytic preparation of chlorine from aqueous solutions of sodium chloride. A detailed description of chloralkali electrolysis using oxygen-consuming cathodes may be found in Journal of Applied Electrochemistry, Vol 38 (9), pages 1177-1194 (2008). Examples of electrolysis cells having oxygen-consuming cathodes may be found in the documents EP 1033419B1, DE 19622744C1 and WO 2008006909A2.
An oxygen-consuming cathode typically consists of a support element, for example a plate composed of porous metal or a woven mesh composed of metal wires, and an electrochemically active coating (DE 3710168). The electrochemically active coating is porous and consists of hydrophilic and hydrophobic constituents. The hydrophobic constituents make the penetration of electrolytes difficult and thus keep the appropriate pores free for transport of the oxygen to the catalytically active sites. The hydrophilic constituents allow penetration of the electrolyte to the catalytically active sites and the outward transport of the hydroxide ions. As hydrophobic component, use is made of, for example, polytetrafluoroethylene (PTFE). Hydrophobic surfaces usually have a contact angle of more than 140° on wetting with pure water.
In the manufacture of oxygen-consuming cathodes, a distinction can be made in principle between dry and wet manufacturing processes.
In the dry processes, a powder mixture of catalyst and polymeric component (e.g. PTFE) is produced (e.g. as described in DE 2941774) and milled to particles which are subsequently distributed on an electrically conductive support element and pressed at room temperature. Such a process is described, for example, in EP 1728896A2. Silver-oxide/silver is mentioned as preferred catalyst, polytetrafluoroethylene (PTFE) is mentioned as binder and a gauze made of nickel wires is mentioned as support.
In the case of wet manufacturing processes, either a paste or a suspension consisting of catalyst and polymeric component in a dispersion medium, preferably water, is used. To produce the suspension, it is possible to add additional surface-active substances in order to increase the stability of the suspension or paste. To improve the processability, a thickener can be added to the suspension. The paste is subsequently applied by screen printing or calendering to a current distributor, while the less viscous suspension is usually sprayed on.
The paste or suspension is dried gently after removal of excess dispersion medium and pressed at temperatures in the region of the melting point of the polymer (Journal of Applied Electrochemistry, Vol 38 (9) pages 1177-1194 (2008)). The oxygen-consuming cathodes can consist of a single layer applied to a support. The support can in this case take on a number of tasks, firstly provision of the mechanical stability of the finished OCC and/or distribution of the current within the catalytically active layer.
However, the known single-layer electrodes have the disadvantage that they are sensitive to breakthrough of liquid or gas. This is of critical importance especially in industrial electrolysers. Here, gas must not get from the gas space into the electrolyte space, and electrolyte must not get from the electrolyte space into the gas space. In industrial electrolysers, the oxygen-consuming cathode must withstand the hydrostatic pressure prevailing at the bottom of the industrial electrolysis cell of, for example, 170 mbar. Since a gas diffusion electrode has a pore system, a small amount of liquid always gets into the gas space and gas gets into the liquid space. The amount depends on the cell design of the electrolyser. The OCC should be gas-impermeable/liquid-impermeable at a pressure difference between the gas space and the liquid space of 10-60 mbar. Here, gas-impermeable means that no entry of gas bubbles into the electrolyte space can be observed with the naked eye. Liquid-impermeable means that an amount of liquid of not more than 10 g/(h*cm2) goes through the OCC (where g is the mass of liquid, h is one hour and cm2 is the geometric electrode surface area).
However, if too much liquid gets through the OCC, this can flow downwards only on the side facing the gas side. This can result in formation of a liquid film which hinders access of the gas to the OCC and thereby has an extremely adverse effect on the performance of the OCC (undersupply of oxygen). If too much gas gets into the electrolyte space, the gas bubbles have to be able to be discharged from the electrolyte space. In any case, the gas bubbles cover part of the electrode area and the membrane area, which leads to a shift in power density and thus in galvanostatic operation of the cell to a local increase in the current density and an undesirable increase in cell voltage over the cell.
Single-layer oxygen-consuming cathodes which satisfy this requirement profile have not been known hitherto. To operate the available oxygen-consuming cathodes in an industrial electrolyser, the cell design has hitherto been adapted to the deficiencies of the oxygen-consuming cathode. For example, electrolysers having pressure compensation, as described in DE 19622744C1, which compensate for the hydrostatic height of the liquid upstream of the electrode by dividing the gas space by means of gas pockets, with a gas pressure adapted to the hydrostatic pressure being set in each gas pocket, have been developed. However, these have the disadvantage that, apart from the high costs and use of material in construction of the cell, electrode area is also lost and a larger number of electrolysis elements or more electrode area is required to achieve the same power of the electrolysis cell.
It has been found that single-layer OCC produced by the wet process, in particular, cannot be used in industrial electrolysers because of unsatisfactory gas-impermeability and electrolyte-impermeability.
Multilayer oxygen-consuming cathodes can fundamentally be produced by the dry or wet pasting process. According to EP 1728896, multilayer oxygen-consuming cathodes are produced by the dry process. Here, powder layers in which the PTFE content varies from 3 to 15% by weight are produced. No information is given about the arrangement of the layers having a differing PTFE content. A disadvantage of the process described is that the layers cannot be made arbitrarily thin. A consequence of this is that a multilayer oxygen-consuming cathode having this structure would be relatively thick, which leads to an increased cell voltage during electrolysis and incurs high production costs due to the increased amount of material.
U.S. Pat. No. 4,602,4261 likewise discloses a multilayer oxygen-consuming cathode produced by the dry process. The best two-layer electrode disclosed has a thickness of at least 0.89 mm. PTFE contents disclosed are in the range from at least 10% by weight to 50% by weight. Such high PTFE contents can lead to the active catalyst particles not forming an electrically conductive network in the electrode and therefore the total amount of catalyst material is no longer available for the electrochemical reaction. This reduces the efficiency of the electrolysis cell and thus increases the cell voltage.
U.S. Pat. No. 5,584,976 discloses a multilayer oxygen-consuming cathode which is produced by applying layers to both sides of a silver substrate. Here, the outermost layer facing the gas side is made completely of a hydrophobic material; a pure, porous PTFE layer is disclosed here. The cell voltages disclosed for these multilayer oxygen-consuming cathodes are in the range from 2.3 to 2.4 V (at a current density of 3 kA/m2) and are therefore very high.
It is common to all these multilayer oxygen-consuming cathodes that either the PTFE contents in the hydrophobic layers are relatively high and thus lead to high cell voltages or the production processes lead to thick electrodes and thus likewise to relatively high cell voltages.
Particularly when the side facing the gas side has a high PTFE content, the electrode cannot be used in particular cell constructions. Thus, for example, the contacting of the OCC in the falling film cell technology (DE 3401636A1 or WO 0157290A1) is effected from the gas side by means of an elastic mat or similar constructions. However, at high PTFE contents, the electrical conductivity of the layer increases greatly, so that either there is a higher ohmic resistance, which leads to a higher cell voltage, or contacting is not possible at all, as a result of which the installation of the OCC in the electrolysis cell is very complicated.
Bidault et al. (2010, Int. J. of Hydrogen Energy, “An improved Cathode for alkaline fuel cells”, 35, pp. 1783-1788) have been able to show in studies that the thickness of the oxygen-consuming cathode can have a very great influence on the overall performance of the electrode. In measurements using two-layer electrodes on a nickel foam, the layer facing the anode consisted of plated-on silver. The layer facing the gas space consisted of a porous carbon/PTFE mixture. These electrodes showed that lowering the thickness from 0.7 mm to 0.5 mm made it possible to improve the half-cell potential by 100 mV. Layer thicknesses of less than 0.5 mm in the two-layer construction used here lead to electrodes which are not sufficiently electrolyte-impermeable and in which electrolyte breakthrough into the gas space occurs.